1. Field of the Invention
The present invention relates generally to an electronic control system using a microcomputer for controlling the idling speed of an internal combustion engine, and more specifically to an electronic control system for controlling engine idling speed in either an open-loop or feedback control mode according to the engine operating condition by adjusting the degree of opening of an auxiliary air control valve (referred hereinafter simply as AAC valve) continuously so as to provide an appropriate intake air flow quantity for the engine.
2. Description of the Prior Art
In recent years, electronic control systems using microcomputers have been applied to automotive vehicle for appropriately controlling the fuel injection rate, ignition spark timing, exhaust gas recirculation, etc. of internal combustion engines.
Since the present invention relates to a system for controlling engine idling speed among other things, a prior art system will now be briefly described.
The systems generally comprises: (a) a control unit, (b) vacuum control modulator valve (referred simply to as VCM valve), and (c) an AAC valve.
The control unit controls an air mixture fuel supplied to the engine, according to input signals from a throttle valve (hereinafter referred to as idle switch) which turns on when the throttle valve is in the idling state, a crank angle sensor, a temperature sensor which senses the temperature of cooling water, a vehicle speed sensor, etc.
The VCM valve controls a vacuum pressure applied to the AAC valve according to an output pulse signal with a duty ratio obtained from the control unit.
The AAC valve controls the intake air flow quantity of an auxiliary air passage according to the controlled vacuum pressure from the VCM valve.
The control unit described above, when providing automatic control over a controlled result, e.g., the number of engine revolutions, detects the conditions under which the engine is being operated to determine whether it should perform feedback control or open-loop control, according to input signals indicating the engine load condition such as a throttle switch, vehicle speed sensor, neutral switch of a transmission gear or crank angle sensor.
Depending on the result of this determination, the control unit outputs a pulse signal, after a predetermined processing of arithmetic operations for obtaining the proper duty ratio for the pulse signal.
In the feedback control mode, the deviation of the actual number of engine revolutions per time (engine speed), measured by the crank angle sensor, from a predetermined number of engine revolutions (reference input) is obtained. If the deviation exceeds a predetermined zone (dead zone), a duty ratio of pulse signal to be fed to the VCM valve is adjusted so as to introduce the instaneous (or actual) number of engine revolutions (engine speed) within the predetermined zone (dead zone). Consequently, the VCM valve actuates the AAC valve to open an amount to provide an appropriate intake air flow quantity to maintain the instantaneous number of engine revolutions within the predetermined zone.
The repetitions of such cycle in the feedback control mode are performed so that the instantaneous number of engine idling revolutions (controlled variable: engine speed) settles within the predetermined zone. On the other hand, in the open-loop control mode, a numerical value stored in a memory of the control unit is read out to provide the duty ratio of the output pulse signal according to the engine operating condition, e.g., a cooling water temperature for the engine. The control unit can roughly be divided into two circuits: a control mode determining circuit and arithmetic and logic operation/memory circuit.
In operation, the control unit checks to see whether the idle switch is turned on or not. If the idle switch is turned off, the control unit executes open-loop control. If the idle switch is turned on, the control unit further checks to see whether the instantaneous number of engine revolutions obtained from the crank angle sensor is below the predetermined zone (dead zone: the minimum limit may be the reference input value minus 25 rpm). If the engine speed is below the reference value, the control unit performs feedback control immediately in the next step. If engine speed is above a reference value, the control unit checks to see whether the elapsed time from the time when the throttle valve switch is turned on is more than 4 sec. If it is found not more than 4 sec., the control unit continues open-loop control. If it is found more than 4 sec., the control unit advances to the next step where the control unit checks to see whether the elapsed time from the time when the neutral switch of the transmission gear is turned on is more than 1 sec. If it is more than 1 sec., the control unit switches and execute the feedback control. If it is not more than 1 sec., the control unit checks to see whether the elapsed time from the time when the vehicle speed decreases and arrives at 8 Km/h is more than 1 sec. If it is found not more than 1 sec., the control unit continues open-loop control. If it is found more than 1 sec., the control unit switches and executes the feedback control.
In such a conventional system for controlling the engine idling speed the fixed time delay described above is provided to start the actual feedback control. Therefore, when engine conditions indicate feedback control operation, actual feedback control may be started earlier than desired if the engine idling speed is excessively high with respect to the predetermined zone (dead zone) even after elapsing the fixed delay time at the time when the idle switch is turned on with the transmission gear in the neutral position. Consequently, an undershooting of the output engine speed occurs and the engine idling speed may drop abruptly and even cause engine stalling in a worst case.
In addition, the engine idling speed may generally be set at a lower range to improve fuel consumption savings. However, as the engine idling speed is reduced, the stability of the engine (controlled device) will be reduced in proportion thereto. For this reason, if the engine idling speed is set lower, when an abrupt change of the intake air flow quantity (manipulated variable of controlled device) occurs at the instant when control is transferred from open-loop to feedback control the engine speed will not settle smoothly to a predetermined speed since the controlled variable and manipulated variable are not in a steady state. Consequently, an unfavorable hunting or engine stalling may occur due to the abrupt speed drop in the predetermined engine idling speed.